Technical Field
This disclosure generally relates to methods and systems of driving light emitting diodes (“LEDs”). More particularly, the present disclosure relates to LED driver circuits that maintain an input reference level for an LED driver power stage.
Description of Related Art
An LED is a P-N junction diode that emits light when a suitable voltage is applied to its leads. To that end, various circuits are used to power an LED. Such circuits not only provide sufficient current to light the LED at the desired brightness and color temperature, but also limit the current to prevent damaging the LED. FIG. 1A illustrates an example of a prior art LED driver circuit 100 that regulates output current 101 to LEDs 115 at a level indicated by a control signal at a control signal input 103 when a pulse width modulation (“PWM”) signal at the PWM node 105 is ON (i.e., HI). When the PWM signal is OFF, the output current 101 is zero and the LED load 115 emits no light. Hence, the average value of the output current 101 is controlled by the relative ON and OFF durations of the PWM signal. Put differently, the intensity of the light emitted by the LEDs 115 can be increased with a higher duty cycle and dimmed by lowering the duty cycle of the PWM signal at node 105.
As illustrated in FIG. 1A, an LED driver circuit 100 may include an error amplifier 107 having a control signal input 103, two electronic switches (i.e., the first switch 109 and the second switch 111), an operating point capacitance element 113, an optional output capacitance 117, an LED driver power stage 119, and a current sensor 121.
The error amplifier 107 compares the control input signal at the control signal input node 103 with the output current 101 sensed by the current sensor 121 to generate a signal at its output node 123. This signal 123 provides an operating point signal (e.g., voltage Vc) when the switch 109 is ON. The error amplifier 107 adjusts the operating point to reduce the error signal between the control signal input 103 and a voltage representation of the current that is flowing through the LED load 115. The voltage at the operating point signal node Vc is used by the LED driver power stage 119 to set the amount of output current 101 that is delivered to the LEDs 115. Thus, the signal 123 at the output of the error amplifier provides an operating point for the LED driver circuit 119 for the amount of output current 101 to match the amount indicated by the control signal at the control signal input 103 of the error amplifier 107.
The capacitance element 113 may therefore be referred to as an operating point capacitance because the voltage across it (i.e., the operating point signal) represents the input operating point signal to the LED driver power stage 119 that is used to cause the output current 101 to the LEDs 115 to be equal to the amount indicated by the control signal 103. The operating point capacitance element 113 stores the operating point signal of node Vc for the LED driver circuit 119. Thus, the capacitance element 113 stores the operating point of the power stage 119 such that the current in the LED load 115 is regulated to the CTRL input 103 of the error amplifier 107. The capacitance element 113 may also function to stabilize the LED current feedback control loop. In this regard, the capacitance of the capacitance element 113 may be limited in maximum value.
The features of the LED driver circuit 100 may be better understood in view of FIG. 1B, which illustrates some example waveforms of the LED driver circuit 100. Ideally, the capacitance element 113 should hold the voltage of the operating point signal Vc when the switch 109 is OFF (i.e., open), to keep the operating point signal Vc stable for the LED driver power stage 119. However, under real world conditions, the voltage across the operating point capacitance element 113 decays (i.e., loses charge) during OFF periods of the PWM signal at node 105 due to internal leakage and/or leakage of any circuits connected to the operating point capacitance element 113, including the first switch 109. The voltage drop becomes more significant as the PWM OFF duration increases. After a long PWM OFF time (e.g., more than 1 second), for example, the operating point signal Vc across the operating point capacitance element 113 may be lower than its value when (e.g., just after) the PWM signal is turned OFF. Put differently, the value of the operating point signal Vc is higher at the transition point when the PWM is turned OFF, than the value after a long PWM OFF time. When the PWM signal 105 is turned back ON after a long PWM OFF period, the LED driver power stage 119 may be subject to a recovery time until the voltage across the operating point capacitance element 113 has returned to its original operating point signal Vc.
Such a delay can be problematic in applications that desire the color temperature and/or the intensity of the LEDs 115 to be at a predetermined level immediately after they are turned ON. Traditional approaches of having longer PWM ON time to include the recovery delay in addition to the desired LED load ON time not only increases power consumption but may not be effective because the recovery delay may vary with the size of the operating point capacitance element 113, process, temperature, desired LED light intensity, and the PWM OFF durations.